I can't really imagine any statement they could give that would ease concerns that at some point in time they change their mind. But for now, it is a relief to read, even if this is a bit of marketing. The longer it goes without being enshittified the better.
They could agree to some actual significant negative consequence to running ads. e.g. They could put a clause in the subscription signup process that says if they ever run ads - even if it's only for free accounts - then you get all of the money you've spent back.
Of course I realise they would never do something like that. Buy why not? Well, because they might decide they want to run ads...